There are some important things about Ice Ages that we don’t understand. These include:
We don’t understand the changes in the amplitude and period of oscillations over the past 3 million years, as shown in Fig. 2.2.
We don’t understand the occurrence of rapid changes in Greenland temperatures during Ice Ages.
We do understand how the solar power input to high northern latitudes changes with time over long time-periods. This pattern is dominated by the 22,000-year period due to precession, and its amplitude is governed by obliquity and eccentricity. We have good data to show that as Ice Ages evolve, the rate of growth of the ice sheets increases when the solar input is low, and vice versa. The solar input acts as a pacemaker to regulate the pattern of ice volume versus time during an Ice Age. But the Ice Ages persist through the ups and downs of several 22,000-year precession cycles; the ice sheets relentlessly advance. Over the last 800,000 years, we observe that at 9 instances, spaced by many tens of thousands of years, the gradual (but sometimes bumpy) expansion of the ice sheets came to an abrupt halt (a termination); the ice sheets disappeared (or were greatly reduced) in a relatively short time (~5500 years). After a warm period with minimal ice that we call an Interglacial, an incipient new Ice Age begins, and the pattern reproduces itself. The evidence seems to suggest that during this 800,000-year period, prior to large-scale human activity, the natural state of the Earth was an Ice Age. A perennial Ice Age would have persisted for the entire 800,000 years except for the fact that in nine instances, something occurred that caused the ice sheets to greatly diminish, but those events were temporary; the ice sheets began rebuilding perhaps ~10,000 years later. For some reason unexplainable, the community of scientists studying Ice Ages mainly did not concern itself with terminations. We note the following facts:
Every one of the nine terminations occurred at the start of an up-lobe of the solar input as governed by precession. See Fig. 11.3.
Many of the up-lobes in the solar input as governed by precession, did not produce a termination.
Only the up-lobes that were associated with a termination, were preceded very large, short-term levels of dust recorded in ice cores, suggesting that the ice sheets were covered by dust in those instances.
These facts suggest that an up-lobe in the solar input as governed by precession is necessary for a termination to occur, but not sufficient. Only when the up-lobe is preceded by large amounts of dust deposition, does a termination occur.